What does stereo effect means?

Wire’s stereo feature puts each call participant in a “virtual space.” It creates the illusion that you are in a physical room with the people you are talking to. The result is that participants keep track of who's speaking with more ease and maintain an easier conversational flow.

When multiple participants talk at the same time, it’s possible to distinguish each speaker and even focus more on one voice vs. background noise coming from one of the participants (vs. mono there is a jumble of voices and noise that cannot be discerned).